The Degradation of Serotonin: Role of MAO

Molecular Design of SERTlight: A Fluorescent Serotonin Probe for Neuronal Labeling in the Brain

The serotonergic transmitter system plays fundamental roles in the nervous system in neurotransmission, synaptic plasticity, pathological processes, and therapeutic effects of antidepressants and psychedelics, as well as in the gastrointestinal and circulatory systems. We introduce a novel small molecule fluorescent agent, termed SERTlight, that specifically labels serotonergic neuronal cell bodies, dendrites, and axonal projections as a serotonin transporter (SERT) fluorescent substrate. SERTlight was developed by an iterative molecular design process, based on an aminoethyl-quinolone system, to integrate structural elements that impart SERT substrate activity, sufficient fluorescent brightness, and a broad absence of pharmacological activity, including at serotonin (5-hydroxytryptamine, 5HT) receptors, other G protein-coupled receptors (GPCRs), ion channels, and monoamine transporters. The high labeling selectivity is not achieved by high affinity binding to SERT itself but rather by a sufficient rate of SERT-mediated transport of SERTlight, resulting in accumulation of these molecules in 5HT neurons and yielding a robust and selective optical signal in the mammalian brain. SERTlight provides a stable signal, as it is not released via exocytosis nor by reverse SERT transport induced by 5HT releasers such as MDMA. SERTlight is optically, pharmacologically, and operationally orthogonal to a wide range of genetically encoded sensors, enabling multiplexed imaging. SERTlight enables labeling of distal 5HT axonal projections and simultaneous imaging of the release of endogenous 5HT using the GRAB5HT sensor, providing a new versatile molecular tool for the study of the serotonergic system.

Role of Serotonylation and SERT Posttranslational Modifications in Alzheimer's Disease Pathogenesis

The neurotransmitter serotonin (5-hydroxytryptamine, 5-HT) is implicated mainly in Alzheimer's disease (AD) and reported to be responsible for several processes and roles in the human body, such as regulating sleep, food intake, sexual behavior, anxiety, and drug abuse. It is synthesized from the amino acid tryptophan. Serotonin also functions as a signal between neurons to mature, survive, and differentiate. It plays a crucial role in neuronal plasticity, including cell migration and cell contact formation. Various psychiatric disorders, such as depression, schizophrenia, autism, and Alzheimer's disease, have been linked to an increase in serotonin-dependent signaling during the development of the nervous system. Recent studies have found 5-HT and other monoamines embedded in the nuclei of various cells, including immune cells, the peritoneal mast, and the adrenal medulla. Evidence suggests these monoamines to be involved in widespread intracellular regulation by posttranslational modifications (PTMs) of proteins. Serotonylation is the calcium-dependent process in which 5-HT forms a long-lasting covalent bond to small cytoplasmic G-proteins by endogenous transglutaminase 2 (TGM2). Serotonylation plays a role in various biological processes. The purpose of our article is to summarize historical developments and recent advances in serotonin research and serotonylation in depression, aging, AD, and other age-related neurological diseases. We also discussed several of the latest developments with Serotonin, including biological functions, pathophysiological implications and therapeutic strategies to treat patients with depression, dementia, and other age-related conditions.

Genetic contributions to brain serotonin transporter levels in healthy adults

The serotonin transporter (5-HTT) critically shapes serotonin neurotransmission by regulating extracellular brain serotonin levels; it remains unclear to what extent 5-HTT levels in the human brain are genetically determined. Here we applied [11C]DASB positron emission tomography to image brain 5-HTT levels and evaluated associations with five common serotonin-related genetic variants that might indirectly regulate 5-HTT levels (BDNF rs6265, SLC6A4 5-HTTLPR, HTR1A rs6295, HTR2A rs7333412, and MAOA rs1137070) in 140 healthy volunteers. In addition, we explored whether these variants could predict in vivo 5-HTT levels using a five-fold cross-validation random forest framework. MAOA rs1137070 T-carriers showed significantly higher brain 5-HTT levels compared to C-homozygotes (2-11% across caudate, putamen, midbrain, thalamus, hippocampus, amygdala and neocortex). We did not observe significant associations for the HTR1A rs6295 and HTR2A rs7333412 genotypes. Our previously observed lower subcortical 5-HTT availability for rs6265 met-carriers remained in the presence of these additional variants. Despite this significant association, our prediction models showed that genotype moderately improved prediction of 5-HTT in caudate, but effects were not statistically significant after adjustment for multiple comparisons. Our observations provide additional evidence that serotonin-related genetic variants modulate adult human brain serotonin neurotransmission.

GPR35 and mediators from platelets and mast cells in neutrophil migration and inflammation

Neutrophil recruitment from circulation to sites of inflammation is guided by multiple chemoattractant cues emanating from tissue cells, immune cells, and platelets. Here, we focus on the function of one G-protein coupled receptor, GPR35, in neutrophil recruitment. GPR35 has been challenging to study due the description of multiple ligands and G-protein couplings. Recently, we found that GPR35-expressing hematopoietic cells respond to the serotonin metabolite 5-hydroxyindoleacetic acid (5-HIAA). We discuss distinct response profiles of GPR35 to 5-HIAA compared to other ligands. To place the functions of 5-HIAA in context, we summarize the actions of serotonin in vascular biology and leukocyte recruitment. Important sources of serotonin and 5-HIAA are platelets and mast cells. We discuss the dynamics of cell migration into inflamed tissues and how multiple platelet and mast cell-derived mediators, including 5-HIAA, cooperate to promote neutrophil recruitment. Additional actions of GPR35 in tissue physiology are reviewed. Finally, we discuss how clinically approved drugs that modulate serotonin uptake and metabolism may influence 5-HIAA-GPR35 function, and we speculate about broader influences of the GPR35 ligand-receptor system in immunity and disease.

Exploring the Role of Serotonin as an Immune Modulatory Component in Cardiovascular Diseases

Serotonin, also known as 5-hydroxytryptamine (5-HT) is a well-known neurotransmitter in the central nervous system (CNS), but also plays a significant role in peripheral tissues. There is a growing body of evidence suggesting that serotonin influences immune cell responses and contributes to the development of pathological injury in cardiovascular diseases, such as atherosclerosis, as well as other diseases which occur as a result of immune hyperactivity. In particular, high levels of serotonin are able to activate a multitude of 5-HT receptors found on the surface of immune cells, thereby influencing the process of atherosclerotic plaque formation in arteries. In this review, we will discuss the differences between serotonin production in the CNS and the periphery, and will give a brief outline of the function of serotonin in the periphery. In this context, we will particularly focus on the effects of serotonin on immune cells related to atherosclerosis and identify caveats that are important for future research.

α-Methyltryptamine (α-MT) Metabolite Profiling in Human Hepatocyte Incubations and Postmortem Urine and Blood

α-MT is a hallucinogenic and stimulant tryptamine that was involved in several overdose fatalities in the United States and Europe. Analytical toxicology, and particularly the identification of metabolite biomarkers in biological samples, often is the only way to prove tryptamine use in clinical and forensic caseworks. We aimed to identify optimal α-MT metabolite biomarkers of consumption in humans. We identified α-MT metabolites in 10-donor-pooled human hepatocyte incubations and postmortem urine and blood from an α-MT overdose case using in silico metabolite predictions, liquid chromatography high-resolution-tandem mass spectrometry (LC-HRMS/MS), and software-assisted data mining. Nine metabolites were identified in vitro and eight additional metabolites were found in urine; five metabolites were found in blood. Metabolic transformations were hydroxylation, O-sulfation, O-glucuronidation, N-glucuronidation, and N-acetylation, consistent with the metabolism of structural analogues. The findings in hepatocyte incubations and postmortem samples were consistent, proving the in vitro model suitability. We suggest α-MT, hydroxy-α-MT glucuronide, and two hydroxy-α-MT sulfates as biomarkers of α-MT use in non-hydrolyzed urine; we suggest α-MT, two hydroxy-α-MT sulfates and N-acetyl-α-MT as biomarkers of α-MT use in blood. Further studies on α-MT clinical and forensic caseworks with different doses and routes of administration are necessary to better explore α-MT metabolism.

Biomarkers of 4-hydroxy-N,N-methylpropyltryptamine (4-OH-MPT) intake identified from human hepatocyte incubations

BACKGROUND

4-Hydroxy-N,N-methylpropyltryptamine (4-OH-MPT) is a psychedelic tryptamine whose use is regulated in several countries. Due to unspecific effects, consumption can be ascertained only through toxicological analyses. However, the trace amounts of tryptamines are usually challenging to detect in biological samples. 4-OH-MPT metabolism was characterized to identify optimal metabolite markers of intake in clinical/forensic toxicology.

RESEARCH DESIGN AND METHODS

4-OH-MPT was incubated with 10-donor-pooled human hepatocytes to simulate in vivo conditions; samples were analyzed by liquid chromatography-high-resolution tandem mass spectrometry (LC-HRMS/MS), and data were processed with Compound Discoverer from Thermo Scientific. LC-HRMS/MS and data mining were supported by in silico metabolite predictions (GLORYx).

RESULTS

Three phase I and four phase II metabolites were identified, including N-oxidation and N-demethylation at the alkylamine chain, and O-glucuronidation and sulfation at the hydroxylindole core.

CONCLUSIONS

4-OH-MPT metabolic fate was consistent with the human metabolism of tryptamine analogues: we suggest 4-OH-MPT-N-oxide and 4-hydroxy-N,N-propyltryptamine (4-OH-PT) as metabolite biomarkers of 4-OH-MPT consumption after glucuronide/sulfate hydrolysis in biological samples to improve detection of 4-OH-MPT and phase I metabolites; 4-OH-MPT-glucuronide is suggested as an additional biomarker when hydrolysis is not performed. Further research on the metabolism of structural analogues is necessary to evaluate the specificity of 4-OH-MPT metabolite biomarkers.

Photoperiodic Changes in Both Hypothalamus Neurotransmitters and Circulating Gonadal Steroids Metabolomic Profiles in Relation to Seasonal Reproduction in Male Quail

Both hypothalamic neurotransmitters and serum steroid hormones are impacted by photoperiod and have effects on physiology and seasonal reproductive. However, the relationship between circulating gonadal steroids and hypothalamic neurotransmitters underlying different photoperiod is still unclear. To further understand the crosstalk of neurotransmitters and steroids in seasonal reproduction, metabolic changes of 27 neurotransmitters concentrated in hypothalamus tissues and 42 steroids hormones in serum were assessed during two artificial photoperiodic programs. The results showed that photoperiod induce testicular atrophy and recrudescence. In L-to-S groups, significantly decreased levels of testosterone concentration were found in serum (P < 0.001) and increased 11-Dehydrocorticosterone (P < 0.05); Testosterone were almost undetectable at SD_14d. In addition, the hypothalamus exhibited significantly increased arginine and 4-aminobutyric acid (GABA) concentration and decreased serotonin and epinephrine content (P < 0.01 or P < 0.05). Accordingly, serum testosterone and androstenedione became detectable at LD_3d in the S-to-L group and were markedly increase at LD_7d. Furthermore, Serum androstenedione showed a significant increase with long light expose (P < 0.01). Additionally, the hypothalamus exhibited both significantly increased L.Tryptophan and phenylalanine concentration, as well as decreased L-glutamine and L-glutamine.acid content (P < 0.01 or P < 0.05). Serotonin metabolism showed significant differences between L-to-S group and S-to-L group. Furthermore, in the correlation analysis, serum testosterone had a positive correlation with 5-Hydroxyindole-3-acetic acid (5-HIAA), while Androstenedione was significantly negative with L.Tryptophan in L-to-S (P < 0.05). However, in S-to-L group, serum testosterone showed strong negative correlation with both serotonin and 5-HIAA (P < 0.05), but positive correlation with L.Tryptophan (P < 0.01), while Androstenedione was significantly negative correlation with both serotonin (P < 0.05) and L-Glutamine (P < 0.01). Photoperiod also had significant effects on the mRNA expression. We found significant differences in gene expression patterns of both serotonin signaling and steroid biosynthesis, while MAOB, NR5A1, and 3β-HSD showed an opposite tendency between two groups. Taken together, our results revealed that circulating gonadal steroids and hypothalamic neurotransmitters were significantly impact quail’s seasonal reproduction. Circulating gonadal steroids have different effects on neurotransmitter at different photoperiodism, which may coordinately influence the seasonal reproduction of quails.

Investigating the potential of fish oil as a nutraceutical in an animal model of early life stress

Background: Early life stress is a key predisposing factor for depression and anxiety disorders. Selective serotonin re-uptake inhibitors (SSRI) are frequently used as the first line of pharmacology treatment for depression but have several negative qualities, i.e. a delay or absence of effectiveness and negative side-effects. Therefore, there is a growing need for new nutraceutical-based strategies to blunt the effects of adverse-life events.Objectives: This study aimed to use the maternal separation model in rats to test the efficacy of fish oil dietary supplementation, on its own and in conjunction with the SSRI anti-depressant fluoxetine, as a treatment for depressive and anxiety-like symptoms associated with early life stress.Methods: Behavioural tests (open field test, elevated plus maze test and forced swim test) and biochemical markers (corticosterone, BDNF, brain fatty acids and short chain fatty acids) were used to analyse the effects of the dietary treatments. Gut microbial communities and relating metabolites (SCFA) were analysed to investigate possible changes in the microbiota-gut-brain axis.Results: Maternally separated rats showed depressive-like behaviours in the forced swim and open field tests. These behaviours were prevented significantly by fluoxetine administration and in part by fish oil supplementation. Associated biochemical changes reported include altered brain fatty acids, significantly lower plasma corticosterone levels (AUC) and reduced brain stem serotonin turnover, compared to untreated, maternally separated (MS) rats. Untreated MS animals had significantly lower ratios of SCFA producers such as Caldicoprobacteraceae, Streptococcaceae, Rothia, Lachnospiraceae_NC2004_group, and Ruminococcus_2, along with significantly reduced levels of total SCFA compared to non-separated animals. Compared to untreated MS animals, animals fed fish oil had significantly higher Bacteroidetes and Prevotellaceae and reduced levels of butyrate, while fluoxetine treatment resulted in significantly higher levels of Neochlamydia, Lachnoclostridium, Acetitomaculum and Stenotrophomonas and, acetate and propionate.Conclusion: Despite the limitations in extrapolating from animal behavioural data and the notable differences in pharmacokinetics between rodents and humans, the results of this study provide a further advancement into the understanding of some of the complex systems within which nutraceuticals and pharmaceuticals effect the microbiota-gut-brain axis.

The role of uptake and degradation in the regulation of peripheral serotonin dynamics in Gulf toadfish, Opsanus beta

The neurotransmitter serotonin (5-hyroxytryptamine, 5-HT) is involved in a variety of peripheral processes. Arguably most notable is its role as a circulating vasoconstrictor in the plasma of vertebrates. Plasma 5-HT is maintained at constant levels under normal conditions through the processes of cellular uptake, degradation, and excretion, known collectively as clearance. However, the degree to which each individual component of clearance contributes to this whole animal response remains poorly understood. The goal of this experiment was to determine the extent to which transporter-mediated uptake and intracellular degradation contribute to 5-HT clearance in the model teleost Gulf toadfish (Opsanus beta). Fish that were treated with the 5-HT transport inhibitors fluoxetine, buproprion, and decynium-22 had 1.47-fold higher plasma 5-HT concentrations and a 40% decrease in clearance rate compared to control fish. In contrast, fish treated with the MAO inhibitor clorgyline had a 1.54-fold increase in plasma 5-HT with no change in clearance rate. The results show that transporter-mediated 5-HT uptake plays an important role in controlling circulating 5-HT and whole body 5-HT homeostasis.

What We Know and What We Need to Know about Aromatic and Cationic Biogenic Amines in the Gastrointestinal Tract

Biogenic amines derived from basic and aromatic amino acids (B/A-BAs), polyamines, histamine, serotonin, and catecholamines are a group of molecules playing essential roles in many relevant physiological processes, including cell proliferation, immune response, nutrition and reproduction. All these physiological effects involve a variety of tissue-specific cellular receptors and signalling pathways, which conforms to a very complex network that is not yet well-characterized. Strong evidence has proved the importance of this group of molecules in the gastrointestinal context, also playing roles in several pathologies. This work is based on the hypothesis that integration of biomedical information helps to reach new translational actions. Thus, the major aim of this work is to combine scientific knowledge on biomolecules, metabolism and physiology of the main B/A-BAs involved in the pathophysiology of the gastrointestinal tract, in order to point out important gaps in information and other facts deserving further research efforts in order to connect molecular information with pathophysiological observations.

Role of serotonin in fish reproduction

The neuroendocrine mechanism regulates reproduction through the hypothalamo-pituitary-gonadal (HPG) axis which is evolutionarily conserved in vertebrates. The HPG axis is regulated by a variety of internal as well as external factors. Serotonin, a monoamine neurotransmitter, is involved in a wide range of reproductive functions. In mammals, serotonin regulates sexual behaviors, gonadotropin release and gonadotropin-release hormone (GnRH) secretion. However, the serotonin system in teleost may also play unique role in the control of reproduction as the mechanism of reproductive control in teleosts is not always the same as in the mammalian models. In fish, the serotonin system is also regulated by natural environmental factors as well as chemical substances. In particular, selective serotonin reuptake inhibitors (SSRIs) are commonly detected as pharmaceutical contaminants in the natural environment. Those factors may influence fish reproductive functions via the serotonin system. This review summarizes the functional significance of serotonin in the teleosts reproduction.

Monoamine oxidase A and A/B knockout mice display autistic-like features

Converging lines of evidence show that a sizable subset of autism-spectrum disorders (ASDs) is characterized by increased blood levels of serotonin (5-hydroxytryptamine, 5-HT), yet the mechanistic link between these two phenomena remains unclear. The enzymatic degradation of brain 5-HT is mainly mediated by monoamine oxidase (MAO)A and, in the absence of this enzyme, by its cognate isoenzyme MAOB. MAOA and A/B knockout (KO) mice display high 5-HT levels, particularly during early developmental stages. Here we show that both mutant lines exhibit numerous behavioural hallmarks of ASDs, such as social and communication impairments, perseverative and stereotypical responses, behavioural inflexibility, as well as subtle tactile and motor deficits. Furthermore, both MAOA and A/B KO mice displayed neuropathological alterations reminiscent of typical ASD features, including reduced thickness of the corpus callosum, increased dendritic arborization of pyramidal neurons in the prefrontal cortex and disrupted microarchitecture of the cerebellum. The severity of repetitive responses and neuropathological aberrances was generally greater in MAOA/B KO animals. These findings suggest that the neurochemical imbalances induced by MAOA deficiency (either by itself or in conjunction with lack of MAOB) may result in an array of abnormalities similar to those observed in ASDs. Thus, MAOA and A/B KO mice may afford valuable models to help elucidate the neurobiological bases of these disorders and related neurodevelopmental problems.